This proposal describes experiments using the tools of mouse genetics to determine the in vivo functions of the c-abl proto-oncogene. The c-abl gene product is a cytoplasmic tyrosine kinase important for signal transduction and control of the cell cycle. Knock-out mice deficient in c- abl, generated in the previous funding period, exhibit a number of phenotypes: perinatal lethality, runting, bone abnormalities, and defects in early lymphoid cell lineages, notably a sensitivity to apoptotic stimuli, Several genetic studies of these mice are proposed: First, the osteoporosis causes by the c-abl deficiency will be examined; osteoblasts will be cultured from mutant and control animals, and tested for their ability to respond to a panel of growth factors. Second, new alleles of c- abl will be generated by gene targeting ("knock-in" experiments). One of these alleles will permit the conditional deletions of the gene at selected times in development and in selected tissues; others will express altered forms of c-abl lacking particular domains. Examination of these mice should help determine the functions of each pathway emanating from c- abl. Third, breeding will be used to generate compound mutant mice lacking c-abl and other genes with related functions, including the abi-1 and -2 genes, encoding Abl-interacting proteins, and ret-, a transmembrane receptor kinase under investigation by the Constantini laboratory. Finally, a germ-line mutation of a new member of the Abi family, a mammalian homologue of the yeast Cdc15 gene, will be generate by gene targeting, and the effects of the mutation alone and with other knock-out mutations will be characterize. These studies should help define the many diverse functions of c-abl in mammalian development and physiology.